According to a process of DE-C-29 04 518, .alpha.,.omega.-C.sub.4 -C.sub.20 -alkenols having purities of above 85% are synthesized by catalytic dehydration of the corresponding .alpha.,.omega.-C.sub.4 -C.sub.20 -diols using neutral, simple or mixed pyrophosphates of lithium, sodium strontium or barium or mixtures of these compounds as catalyst. The disadvantage of this process is, on the one hand, the formation of significant proportions of alkanols and, on the other hand, the formation of other by-products increases significantly at conversions of &gt;80% and considerably at conversions of .gtoreq.90%.
EP-B-O 195 943 takes up this prior art and teaches a process for preparing pure .alpha.,.omega.-C.sub.4 -C.sub.20 -alkanols having purities of over 90% by catalytic dehydration of the corresponding .alpha.,.omega.-C.sub.4 -C.sub.20 -diols, wherein .alpha.,.omega.-C.sub.4 -C.sub.20 -diols are selectively and partially dehydrated at temperatures of from 300 to 500.degree. C. in the presence of catalysts comprising simple or mixed phosphates of elements of Group II of the Periodic Table doped with alkali metal or alkaline earth metal compounds, where the catalyst is obtained by preparing a catalyst composition by use of alkaline earth metal orthophosphate or alkaline earth metal hydrogen phosphate or by reacting an alkaline earth metal compound with phosphoric acid to give the corresponding phosphate and adding alkali metal or alkaline earth metal compound, if desired shaping this composition, subsequently drying it and then calcining it at temperatures of from 350 to 950.degree. C., to give .alpha.,.omega.-C.sub.4 -C.sub.20 -alkanols having purities of over 90% at conversions of over 90%.
The catalysts used meet technical requirements in terms of selectivity and operating life, but do not offer a satisfactory solution in terms of the mass throughput, i.e. the effective amount of material which can be passed over the catalyst at a reduced residence time, which is important from a production point of view. The measure frequently employed for the effective throughput over the catalyst is the LHSV, which gives the ratio of starting material flow to catalyst volume. In EP-B-O 195 954, LHSVs of from 0.25 to 0.45 ml/(ml.multidot.h) are given for the throughput over the catalyst in the process described.
It is an object of the present invention to achieve a high selectivity linked to a high space-time yield in the gas-phase dehydration of .alpha.,.omega.-alkanediols, specifically 1,6-hexanediol, by selection of a suitable catalyst system.